The HIV microbicide field is dependent upon testing in non-human primates (NHPs) as the only relevant model to study infection. However, the predictive accuracy of NHP studies of efficacy in humans has not been validated and as such the economic value is unknown. Hence, refinement of this model and development of a novel correlate of efficacy in humans that will reduce the potential use of NHPs is key for the global progress of microbicides and specifically of the Microbicide Innovation Program's mission. This proposal addresses these issues by testing the hypothesis that ex vivo tissue explant cultures can provide a potential surrogate of in vivo efficacy through measurement of intra-tissular drug pharmacology and ex vivo infection/protection. This will be investigated using combined expertise in modeling mucosal tissue infection and measurement of antiretroviral (ARV) drug pharmacokinetics and pharmacodynamics in tissue. The proposal will focus on a reverse transcriptase inhibitor, PMPA (tenofovir), and an entry inhibitor, maraviroc, used alone and in combination as candidate microbicides. In the R21 component of the proposal we will demonstrate the robustness of our ex vivo explant models for analysis of pharmacological parameters and ex vivo infection independently of the origin (human or NHP) and the type of mucosa (cervicovaginal or colorectal). This will be investigated through two Specific Aims: 1) to define ex vivo pharmacological dose-responses (pharmacokinetics and pharmacodynamics) in human and rhesus macaque mucosal tissue explants;2) to define whether the viral backbone affects pharmacological correlates of activity. The next step of our proposal in the R33 component will involve validation of the model as a surrogate for prediction of in vivo efficacy of ARV drugs as vaginal and colorectal microbicides. Here the two Specific Aims are: 3) to assess whether activity of drugs titrated in vivo can be predicted with ex vivo challenge models: 4) to correlate ex vivo and in vivo protection and drug dosing in NHPs. The iterative design of the overall proposal will allow us to assess correlates between intra-tissular pharmacological dosing and efficacy at all levels: tissue type, origin of tissue, route of dosing and challenge, and nature of experiment (ex vivo, in vivo). These correlates will define "conversion factors" of microbicides efficacy between the NHP model and in humans, which will be key for the rational development of existing and future candidate microbicides.